As semiconductor devices have scaled to smaller and smaller dimensions, the gate dielectric thickness has continued to shrink. Although further scaling of devices is still possible, scaling of the gate dielectric thickness has almost reached its practical limit with the conventional gate dielectric material, silicon dioxide. Further scaling of silicon dioxide gate dielectric thickness will involve a host of problems: extremely thin layers allow for large leakage currents due to direct tunneling through the oxide. Because such layers are formed literally from a few layers of atoms, exacting process control is required to repeatably produce such layers. Uniformity of coverage is also critical because device parameters may change dramatically based on the presence or absence of even a single monolayer of dielectric material. Finally, such thin layers form poor diffusion barriers to impurities.
Realizing the limitations of silicon dioxide, researchers have searched for alternative dielectric materials which can be formed in a thicker layer than silicon dioxide and yet still produce the same field effect performance. This performance is often expressed as “equivalent oxide thickness.” Although the alternative material layer may be physically thick, it has the equivalent electrical effect of a much thinner layer of silicon dioxide (commonly called simply “oxide”). In some instances, silicon dioxide has been replaced with a silicon-oxy-nitride (SiON).
SiON gate dielectrics are conventionally formed by forming a thin layer of SiO2 and subjecting the SiO2 layer to a nitridation process. This is followed by a thermal anneal performed in oxidizing ambient. However, the thermal anneal tends to denude nitrogen from the top surface portion 102a of the SiON gate dielectric layer 102 as illustrated in FIG. 1. The ideal profile, however, has a nitrogen concentration at the top surface that is either equal to or greater than the nitrogen concentration in the bulk of the film. FIG. 2 is a graph of nitrogen concentration versus depth comparing the actual nitrogen profile after anneal 202 to an ideal nitrogen profile 204. The actual nitrogen profile 202 shows significant nitrogen loss near the surface of the dielectric layer.